1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus including the same, and more particularly, to a zoom lens suited for use in a broadcasting camera, a video camera, a digital still camera, and a silver-halide film camera.
2. Description of the Related Art
In recent years, a zoom lens with a large aperture diameter ratio, a high zoom ratio, and a high optical performance is desired for use in an image pickup apparatus, such as a television camera, a silver-halide film camera, a digital still camera, or a video camera. A positive lead type four-unit zoom lens has been known as the zoom lens with a large aperture diameter ratio and a high zoom ratio. In the four-unit zoom lens, four lens units are provided in total, and one of the lens units located closest to an object side has positive refractive power. It is known that the four-unit zoom lens includes a first lens unit for focusing, a second lens unit having negative refractive power for varying magnification, a third lens unit for correcting an image plane variation due to the varying magnification, and a fourth lens unit having positive refractive power for imaging, which are arranged in the stated order from the object side to the image side.
For instance, Japanese Patent No. 3495772 discloses a zoom lens in which a first lens unit includes a first sub lens unit having negative refractive power, a second sub lens unit having positive refractive power, and a third sub lens unit having positive refractive power, and both the second sub lens unit and the third sub lens unit move to the object side when focusing from an object at infinity to an object at close range.
Further, Japanese Patent No. 3301579 discloses a zoom lens in which a first lens unit includes a first sub lens unit having negative refractive power, a second sub lens unit having positive refractive power, and a third sub lens unit having positive refractive power, and the second sub lens unit and the third sub lens unit move to the image side and the object side, respectively, when focusing from an object at infinity to an object at close range.
In broadcasting of sports events or photography for a nature program, shooting subjects are usually performed from a distance, and hence it is preferred to use a telephoto zoom lens having a high zoom ratio and a long focal length at a telephoto end. On the contrary, along with a higher pixel density of an image pickup element, the telephoto zoom lens is required to have high performance. Particularly, high optical performance is required in the entire zoom region and in the entire focus region. In order to achieve high optical performance in the entire zoom region and in the entire focus region of the above-mentioned positive lead type four-unit zoom lens, it is necessary to appropriately set refractive powers and lens configurations of the lens units. In particular, it is necessary to appropriately set power arrangement and lens configuration in the first lens unit. If the power arrangement and the lens configuration of the first lens unit are not optimized, a fluctuation of spherical aberration or coma aberration increases near a minimum object distance on a telephoto end side, and hence it becomes difficult to achieve higher performance in the entire focus region. In addition, if the number of lenses in the first lens unit is increased so as to increase flexibility on aberration correction for achieving higher performance, the first lens unit becomes thicker and larger.